The present invention relates to a method and a circuit arrangement for controlling the operating current of an electromagnetic valve in response to the application of a variable duration pulse. The invention is particularly suitable for electronic fuel injection.
A number of attempts have been made to improve the operating efficiency and response characteristics of pulse-operated electromagnetic valves such as vehicle's fuel injection valves. According to U.S. Pat. No. 4,180,026, the actuating coils of fuel injection valves are initially energized with a relatively large current to open the valve in quick response to the leading edge of an applied pulse. The current is reduced to a lower level necessary to hold the valve open during the remainder of the pulse duration for purposes of reducing the power consumption and allowing the valve to close in a short period of time in response to the trailing edge of the pulse. The aforesaid U.S. patent employs a switching transistor coupled in series with the actuating coils of the valves and an energy absorbing circuit coupled in parallel with the actuating coils. The switching transistor is turned on in response to the leading edge of an applied pulse and turned off when the current reaches a predetermined value. Upon the turn-off the transistor, a "flywheel" current is generated as a result of the "flywheel" effect of the actuating coils. The energy absorbing circuit allows the flywheel current to pass therethrough to absorb the energy stored in the actuating coils before the valve is switched to the hold mode. However, there is a substantial difference between the peak current value and the holding level and therefore a relatively long transition period elapses between the energization mode and the hold mode. If the valve control pulse terminates during the transition period, a substantial amount of energy still remains to be absorbed, causing the valve to take a relatively long time to return to the closed position.